The present invention relates to the signalling methods used in radio communication networks.
Within a radio communication system, the information to be transmitted is often accompanied by a signal, known in advance to the transmitter and the receiver. Detection of this signal enables the receiver to synchronise with the transmitter (in time and/or frequency). In certain systems, this detection also enables parameters of the radio propagation channel between the transmitter and the receiver to be detected, whereby a coherent demodulation can be applied to the signal in order to extract the information transmitted.
To perform such detection, the receiver computes a time correlation function between the received signal and the known pattern of the transmitted signal. The time synchronisation is characterised by the position of the maximum of the modulus of this correlation function. The correlation function or the value of its maximum modulus provides an observation of the channel response and can be used to estimate this response. Performing correlation computations on differential signals avoids any problems due to possible carrier frequency discrepancies between the transmitter and the receiver, and makes it possible to evaluate these discrepancies in order to correct them.
To transmit signalling information, most systems use control channels on which this information is transmitted or broadcast.
Sometimes, some of the signalling information is supplied by reserving a portion of the signal transmitted on a dedicated communication channel (a few information bits). This method may be used in a xe2x80x9cframe stealingxe2x80x9d mechanism, enabling a logical signalling channel to be inserted in the radio resource supporting a traffic channel. This is used in the GSM system, in particular, to insert a fast associated control channel (FACCH) in a traffic channel. A disadvantage of this method is its adverse effect on the transmission rate available on the traffic channel due to the rate consumed by the reserved signal portion (overhead).
An object of the present invention is to propose a simple method of communicating signalling information in a system using synchronisation patterns between the transmitter and the receiver.
Accordingly, the invention proposes a signalling method in a radio communication system, wherein a first station transmits, toward at least one second station, a radio signal formed from a first baseband signal including a synchronisation signal containing at least one synchronisation pattern selected on the basis of signalling information to be supplied to the second station, wherein the second station obtains a second baseband signal from a received radio signal, detects the synchronisation signal in a portion of the second baseband signal to compute synchronisation parameters and/or parameters for estimating a radio transmission channel and uses the computed parameters to demodulate another portion of the second baseband signal and extract transmitted information therefrom, whereby the second station performs a search of several patterns in the second baseband signal to obtain said signalling information on the basis of the detected pattern.
The signalling information is sent at the same time as the synchronisation patterns, which enhances transmission capacity. Several synchronisation patterns are defined, and the sending of one of them provides the recipient with the signalling information. A method of this type is described in WO99/00927.
If the number of synchronisation patterns is 2N, the method enables N bits of signalling information to be transmitted whenever a pattern occurs in the signal. Accordingly, N bits of signalling information can be transmitted by means of N synchronisation patterns successively transmitted, each transmitted pattern being chosen from two predetermined patterns.
The different synchronisation patterns which may be used must have good auto-correlation and cross-correlation properties in order to limit the risks of detection errors. In other words, the auto-correlation function ACx(t) of each pattern x has a pronounced maximum for a zero time offset (ACx(0) greater than  greater than ACx(t) for txe2x89xa00 and the cross-correlation function CCx,x=(t) of two distinct patterns x, xxe2x80x2 is lower than the auto-correlation of any one of these patterns by at least one order of magnitude (CCx,x,(t) less than  less than ACx(0)).
According to the invention, the first baseband signal is a complex signal and the synchronisation pattern is selected from a set of patterns containing a first pattern x, whose real and imaginary parts xr, xi have substantially identical auto-correlations and a cross-correlation substantially lower than said auto-correlations (CCxr,xi(t) less than  less than ACxr(0)≈ACxi(0) for any t) and a second complex pattern x* which is conjugate to the first pattern.
The fact of having two synchronisation patterns where one is a conjugate complex of the other permits a simplification of the means needed at the receiver to extract the signalling information. Indeed, the computations of the elementary correlation functions of the real and imaginary parts of the second baseband signal with the real and imaginary parts of one of the two conjugate patterns, needed to evaluate the complex correlation of the second baseband signal with this pattern, can also be applied directly in the evaluation of the correlation with the other pattern. The auto- and cross-correlation properties of the real and imaginary parts of the synchronisation pattern ensure that this complex pattern and its conjugate will also have good auto- and cross-correlation properties.
If the first baseband signal is a complex signal, generated in response to the flow of digital symbols by a phase modulation such that the modulated phase is anti-symmetrical as a function of the symbols of the flow, a modality or a variant of the above-described embodiment consists in selecting a synchronisation sequence from a predetermined set containing at least one first sequence, giving rise after modulation to a synchronisation pattern whose real and imaginary parts have substantially identical auto-correlations and a cross-correlation substantially lower than said auto-correlations, and a second sequence opposite the first sequence.
This latter method is exactly the same as the first if the synchronisation patterns are transmitted in isolation.
If they are immediately preceded or followed by transmitted information symbols, the synchronisation patterns will not be exactly conjugate complexes of each other if the modulation and/or the propagation channel generates a memory effect causing inter-symbol interference. Nevertheless, they are approximately conjugate complexes and the auto-correlation and cross-correlation properties remain satisfactory if the length of the synchronisation sequences is substantially longer than the memory of the channel and of the modulation. In this case, the receiver may conserve its simplicity by seeking conjugate patterns that are approximately equal to the synchronisation patterns inserted by the transmitter.
To implement the method, the invention proposes, as the xe2x80x9csecond stationxe2x80x9d, a radio communication receiver comprising means for generating a complex baseband signal from a received radio signal, means for detecting a synchronisation signal including at least one synchronisation pattern in a portion of said baseband signal, for computing parameters for synchronising with a transmitter and/or parameters for estimating a radio transmission channel between the transmitter and the receiver, and means for demodulating another portion of said baseband signal with the aid of the computed parameters to extract transmitted information therefrom, the detection means being arranged to search a plurality of synchronisation patterns of a predetermined set of patterns in said baseband signal and obtain signalling information in response to the detection of a pattern of the set and as a function of the detected pattern, said set of patterns containing a first pattern whose real and imaginary parts have substantially identical auto-correlations and a cross-correlation substantially lower than said auto-correlations, and a second pattern which is a conjugate complex of the first pattern.
To implement the method, the invention proposes, as a xe2x80x9cfirst stationxe2x80x9d, a radio communication transmitter comprising means for transmitting, toward at least one receiver, a radio signal formed from a complex baseband signal including a synchronisation signal to enable the receiver to compute synchronisation parameters and/or parameters for estimating a radio transmission channel, means for selecting at least one synchronisation pattern from a predetermined set of patterns on the basis of signalling information to be supplied to the receiver, to include the selected pattern in the synchronisation signal, said set of patterns comprising a first pattern whose real and imaginary parts have substantially identical auto-correlations and a cross-correlation substantially lower than said auto-correlations, and a second pattern which is a complex conjugate of the first pattern.
Alternatively, the radio communication transmitter according to the invention comprises modulation means for generating a complex baseband signal in response to a flow of digital symbols, according to a phase modulation such that the modulated phase is anti-symmetrical as a function of the symbols of the flow, means for transmitting, toward at least one receiver, a radio signal formed from said baseband signal, and means for selecting a synchronisation sequence from a given set of sequences on the basis of signalling information to be supplied to the transmitter, the selected sequence being included in said flow of symbols to generate a synchronisation signal included in said baseband signal to enable the receiver to compute synchronisation parameters and/or parameters for estimating a radio transmission channel, said set of sequences containing at least one first sequence giving rise to a synchronisation pattern whose real and imaginary parts have substantially identical auto-correlations and a cross-correlation substantially lower than said auto-correlations, and a second sequence opposite the first sequence.
Another aspect of the present invention relates to a radio signal repeater comprising means for generating a first baseband signal from a received radio signal, means for detecting a synchronisation signal containing at least one synchronisation pattern generated in response to at least one sequence of synchronisation symbols in a portion of the first baseband signal, to compute parameters for synchronising with a transmitter and/or parameters for estimating a radio transmission channel from said transmitter, means for demodulating another portion of the first baseband signal with the aid of the computed parameters, to generate a flow of demodulated symbols, means for generating a second baseband signal from the flow of demodulated symbols and each sequence of synchronisation symbols detected, means for transmitting, toward at least one receiver, another radio signal formed from the second baseband signal with a frame structure corresponding to a frame structure of the received radio signal, and signalling means for transmitting to the receiver information indicating that the repeater is of the type outlined above, i.e. that it performs demodulation and remodulation of the repeated radio signals without controlling the frame structure of the signals.
These signalling means may operate by inserting signalling symbols at given locations reserved for this purpose in the flow of demodulated symbols. However, these signalling means preferably operate in accordance with the method outlined above, i.e. the means for generating the second baseband signal comprise means for transforming at least one detected sequence of synchronisation symbols, and a modulator to generate the second baseband signal in response to the flow of demodulated symbols with the transformed sequence. Alternatively, the means for generating the second baseband signal may comprise a modulator to generate an intermediate baseband signal in response to the flow of demodulated signals containing the detected sequence, and means for obtaining the second baseband signal by transforming the intermediate baseband signal within a period corresponding to a synchronisation pattern.
The signalling information coded by the received synchronisation pattern therefore provides the receiver with information as to whether such a repeater was or was not inserted in the transmission chain.
In other applications of the method, the transmitted signalling information may identify logical channels sharing the same radio resource or else modulation schemes applied to the transmitted signal frames.